Light-scattering spectroscopy entails illumination of a substance and analyzing light that is scattered at angles relative to the incident source. The photon-matter interactions of the scattering events may be either elastic or inelastic. In an inelastic event, a photon's energy (wavelength) changes as a result of the light-matter interaction. In an elastic event, a photon's energy (wavelength) does not change. Absorption, the phenomena in which a fraction of photons are entirely absorbed, also plays a role in light-scattering spectroscopies.
Raman, diffuse reflectance, and fluorescence spectroscopies are of particular interest as they relate to vibrational and nonvibrational photonic responses of a material. The Raman effect describes a subtle light-matter interaction. Minute fractions of light illuminating a substance are Raman-scattered in random directions. Raman-scattered light is color shifted from the incident beam (usually a laser). The color frequency shifts are highly specific as they relate to molecular bond vibrations inducing molecular polarizability changes. Raman spectroscopy is a powerful technique for chemical analysis and monitoring. Analysis of the resulting low light levels require sophisticated, expensive instrumentation and technical complexity.
Specular reflectance relates to a surface's mirror-like aspects. Diffuse reflectance relates to light that is elastically scattered from the surface of a material at diffuse angles relative to the incident beam. For example, a projector screen diffusely reflects light while a glossy, newly waxed car has a high specular component. Diffuse reflectance spectroscopy is important for chemical analysis as well as measuring visual perception.
Fluorescence relates to substances which absorb light at one wavelength then re-emit it at a longer wavelength as a result of electronic transitions. As an example, a “highlighter” felt-tip marker appears to “glow” green as it absorbs blue and ultraviolet light then emits it as green. Fluorescence provides a powerful technique for chemical monitoring.
Raman spectroscopy involves energizing a sample with a high-power, narrow-wavelength energy source, such as a laser. The laser photons induce low intensity light emissions as wavelengths shift. The Raman effect is an inelastic scattering of photons. The emitted Raman light is collected and analyzed with a specialized instrument.
The spectral positions (colors) of the shifts provide fingerprints of the chemicals in the sample. Thus, Raman spectroscopy provides a means for chemical identification. The intensity of the shift (the spectral peak height) correlates to chemical concentration. Thus, a properly calibrated instrument provides chemical content and concentration. In practicality, Raman spectroscopy is technically complex and requires sophisticated, expensive instrumentation.
The basic concept for a probe-based, on-line Raman instrument is simple. Laser light is directed down an optical fiber to a remote probe. The laser light exits the fiber and illuminates the sample medium. Another fiber picks up the Raman-emitted light and returns it to the instrument for analysis. Optical aspects of probe engineering have historically required particular design finesse. The Raman effect involves very weak signals. Raman emissions may be one trillionth as intense as the exciting radiation. Subsequently, the probe must be incredibly efficient in collecting and transmitting Raman-emitted light. Furthermore, the signal must not be corrupted by extraneous influences.
A band-pass (laser line) filter may be used at the delivery end of a light delivery optical fiber to remove the silica Raman bands arising from the fiber itself before illuminating a sample. A long-pass filter may be disposed before a collection fiber so that only the Stokes scattered light enters the fiber. Filtering for optical fiber-based Raman spectroscopy is described, for example in U.S. Statutory Invention Registration No. H002002. In order to make a filtered probe of the style depicted (not using expanded beam optics), typically each fiber is filtered individually. As disclosed in U.S. Pat. No. 6,222,970, this is generally accomplished by depositing a filter on the fiber end face and butting this filter to another fiber using a tube or coupler to join/align the two fibers. Subsequently, the filtered collection fibers and filtered excitation fiber are epoxied together and placed inside a larger tube, epoxied in place, and then polished so the end face is an optical finish. Alternatively, this can also be accomplished by placing the filter on a glass (fused silica) substrate, machining it to a small circle and placing it between the fibers, as taught in U.S. Pat. No. 5,774,610, and completing as described above.
Since the filters are positioned back from the tip itself this necessitates that the rigid section of the probe be long which limits its usefulness for many applications (such as endoscopic applications), and the complexity of the probe manufacturing process is high, since the more collection fibers used the more time is required to make each filtered fiber. The diameter also becomes larger since each fiber needs a connecting tube, so the size grows with each filtered fiber. If smaller fibers are used they become even more difficult to handle and construct if attempting to make a very small diameter probe. If the filter is deposited on the end face of the fibers and the filter is at the end of the probe the filter can be scratched easily, and aligning all the filtered fibers so the end is smooth and uniform is very difficult and time consuming, because one cannot polish the end since the filters would be removed.